This invention relates to a furnace, and more particularly, to a muffle furnace for firing multilayered ceramic carriers used to mount and interconnect a plurality of integrated circuit chips.
There currently exists furnaces for firing of precious metals, such as gold. The firing of precious metals can be accomplished in the presence of air (i.e., oxygen) since the precious metals will not oxidize, and atmosphere contaminants will not interfere with the firing of the precious metal. No furnace presently exists which has been designed specifically for the firing of thick film pastes of a non-precious metal, in which the film paste of a non-precious metal can also include an organic vehicle material, or binder. (The non-precious metal of the preferred embodiment being copper, which is used in the manufacturing processes of a multilayered substrate for mounting electronic circuit components.)
The industry has tried to use the aforementioned precious metal furnaces for firing of thick film paste of a non-precious metal with little success. Many problems have been encountered; specifically, there is an insufficient exhaust system to evacuate the gases from the burnout zone and there also exists an insufficient fresh gas distribution system in the burnout and firing zones. This is because the organic vehicle contained in the film paste generates substantial amounts of combustion products which needs to be evacuated. The existing furnaces have two exhaust stacks, one placed in the front end of the burnout zone immediately past the entrance curtains (which is essentially a cold zone) and the other located directly above the barrier separating the burnout zone from the firing zone. In existing furnaces, displacement of the burnout exhaust causes the burn off gas to flow against the substrate movement causing reduced microatmospheres above the substrates causing incomplete burnout and other problems such as reduced solderability and loss of adhesion, the substrate(s) being placed on a chain belt which moves through the furnace.
The firing of a thick film paste of a non-precious metal needs to be accomplished in an inert atmosphere. In order to burn off the organic vehicle, oxygen must be used otherwise the organic material carbonizes which can cause short circuits in the substrate. Thus, in the furnace of the present invention a mixture of nitrogen and oxygen is injected into the furnace, but a careful balance needs to exist to insure the non-precious metal (copper) is not oxidized. Another problem with the burn off of the organic material in the existing furnaces is a build up of a carbon residue in the furnace exhaust stack especially since the exhaust stack is in a relative cold zone. After a period of time, the carbon residue, a black tacky substance, starts dropping onto the substrates causing contamination, and thus causing the substrate to be rejected and which in turn requires regular cleaning of the furnace exhaust stack using the "chimney sweep" process, i.e., brushing the inside of the furnace exhaust stack with a large brush.
Thus, there exists a need for a furnace specifically designed for the firing of a thick film paste of a non-precious metal, wherein the film paste can also include an organic vehicle material.